A hitherto unremarked peculiarity of homicide in the United States is that women kill their husbands almost as often as the reverse. For every 100 US. men who kill their wives, about 75 women kill their husbands; this spousal “sex ratio of killing” (SROK) is more than twice that in other Western nations. Our analyses of spousal homicide samples from the United States, Canada, Australia, and Great Britain indicate that this contrast cannot be attributed to greater gun use in the United States, nor to a domain-general convergence of the sexes in their uses of violence. Significant predictors of the spousal SROK include registered versus de facto marriage, coresidency versus separation, ethnicity, and age disparity, but the impacts of these variables are not sufficient to explain the differences between US. and other nations’victim sex ratios.